Terahertz-Raman spectroscopy for in situ benchtop monitoring of changes to extended, supramolecular structure in milling mechanochemistry
Tristan H. Borchers, Filip Topić, Mihails Arhangelskis, M.J. Ferguson, Cameron B. Lennox, Patrick Julien, Tomislav Friščić
Abstract
Low-frequency Raman, also known as terahertz-Raman (THz-Raman), spectroscopy offers a laboratory benchtop-based alternative to synchrotron X-ray diffraction for real-time, in situ monitoring of ball-milling mechanochemical reactions. Although direct monitoring of the long-range structure of materials during mechanochemical reactions is generally challenging by conventional Raman spectroscopy, and typically requires synchrotron X-ray diffraction, here we use THz-Raman spectroscopy to monitor mechanosynthesis of cocrystals, stoichiomorphs, and polymorphs, detect multi-step sequences, and discover solid-state phases in systems difficult to differentiate using fingerprint-region Raman spectroscopy—all through real-time observation of changes in lattice vibrational models. The methodology is augmented by periodic density functional theory (DFT), which enables structural interpretation of spectroscopic changes, notably the identification of THz-Raman bands associated with halogen bond transformations. Simultaneous monitoring of mechanochemical processes in both the fingerprint and low-frequency Raman regions enables real-time observation of changes to extended as well as molecular structure during milling, in a single laboratory benchtop experiment, without synchrotron radiation.